Can you help me with an audio probe question?

[sjaustin]

I have never really used an audio probe until now, so I'm a little tentative with how it works and how to trace the signal path on the schematic. (I've read a few tutorials, trying to apply what I've read.)

I'm troubleshooting an Aion Andromeda. On my test rig, bypass works as expected, but it's just a low level hum when engaged. With the audio probe, I get a loud hum until I touch the probe to the input. Then I get clean signal as expected. I probe one end of R1 and get more clean sound. I probe the other end of R1: nothing. This made me think, wow, the first component in the signal path is the problem, lucky me. But I replaced it and the same thing happens, so maybe the problem is... well, me. 

Schematic attached.

[somnif]

That suggests to me you may have a ground short in there. If your signal has a path to geound, it won't bother visiting your amp on its way there.

Use a DMM and see if you have ground continuity, or at least see what the resistance between that side of R1 and ground looks like.

[jimilee]

That suggests to me you may have a ground short in there. If your signal has a path to geound, it won't bother visiting your amp on its way there.

Use a DMM and see if you have ground continuity, or at least see what the resistance between that side of R1 and ground looks like.

While I agree, R1 is connected to R2 which goes to ground anyway. have you probed anywhere else? Pins 1,2 and 3 on the IC should have audio. Also, have you checked voltages yet?

[sjaustin]

No audio on pins 1–3 of IC2.

Resistance of about 1k between the silent side of R1 and ground.

Voltages hovering around 4.8 on all pins of IC2 except for pin 4 (0.0) and pin 8 (9..

[Rockhorst]

Well, the resistance there should be equal to R2, 1M.

[madbean]

Does your audio probe have a coupling cap? You should have at least 100n between your probe and output to your amp.

[sjaustin]

Does your audio probe have a coupling cap? You should have at least 100n between your probe and output to your amp.

It never occurred to me to check, because I borrowed it from a very experienced builder. But as it turns out: no, it does not. I will put one in and get back to testing soon.

What impact would the absence of the cap have on the function of the probe?

[sjaustin]

I put the cap on the tip of the probe. No change in my results.

[m-Kresol]

the cap is used to decouple any DC offset on the signal. You only want AC signal going into your amp. Any DC would make a huge "thump" at first and could potentially harm speakers and amp. at least that's what I thought it was for.

Your R2 is much too low from your measurement! Check the markings again and look for solder bridges.

As timbo and Rockhorst hinted, with your 1k resistor you basically formed a voltage divider with the major part of the signal going to GND. with the intended 1M your resistance will be higher by a factor of 1000 thus massively limiting the current/signal flow to GND.